The kidney plays a critical role in maintaining physiological homeostasis. Among its homeostatic functions are acid-base balance, regulation of electrolyte concentrations, blood pressure and blood volume regulation. The kidneys accomplish these functions independently, as well as through coordination with other organ systems through the actions of hormones and proteins secreted into the bloodstream. These secreted proteins include erythropoietin (Epo), urodilatin, renin and vitamin D, as well as less emphasized proteins such as adiponectin and leptin.
Adiponectin (also known as AdipoQ, Acrp30, apM1, and GBP28) is an adipocyte-derived cytokine that has been shown to have anti-inflammatory properties. In addition, it functions to regulate blood glucose levels via cross-communication with the liver. Normal blood concentrations of adiponectin are 5-30 μg/ml in humans. Previous studies have shown that circulating levels of adiponectin are elevated during chronic caloric restriction in both humans and mice. In contrast, low levels of adiponectin in human plasma correlate with high insulin, glucose, and triglycerides, as well as increased obesity. It has been shown that over-expression of human adiponectin in transgenic mice resulted in suppression of fat accumulation and prevention of premature death by a high-calorie diet. Furthermore, a diabetes susceptibility locus has been mapped to chromosome 3q27, the location of the human adiponectin gene. Increasing adiponectin blood levels could have therapeutic value in treating diabetes and related comorbidities.
Leptin is a 16-kilodalton-protein hormone that plays a key role in regulating energy intake and energy expenditure by decreasing appetite and increasing metabolism. Recently, leptin has been shown to play a role in protecting the kidneys from renal injury in a mouse model of diabetic nephropathy. In addition, leptin promotes angiogenesis by up-regulating vascular endothelial growth factor.
For over thirty years, erythropoietin (Epo), a 30.4 kDa protein synthesized and secreted mainly by the kidney, has been successfully used to stimulate erythropoiesis in patients suffering from anemia. Recently, it has become apparent that the beneficial effects of EPO extend well beyond the stimulation of red blood cell production (Brines et al., Kidney Int., 2006; 70(2):246-250). Previous studies by Chong et al. established that Epo protects the vascular endothelium against ischemic injury. Chong et al., Circulation, 2002; 106 (23):2973-9. Others have confirmed these findings, demonstrating that Epo has a protective effect on endothelial cells in diverse animal models of vascular disease (Santhanam et al., Stroke, 2005; 36 (12):2731-7; Satoh et al., Circulation, 2006; 113(11):1442-50; Urao et al., Circulation Research, 2006; 98(11):1405-13). In chronic renal failure, patients develop anemia due to inadequate Epo production by the kidney. Recombinant Epo, administered as a replacement therapy, restores hematocrit and blood hemoglobin concentrations, eliminating the need for blood transfusions. This treatment, however, entails regular injections of Epo, two to four times per week, given either intravenously or subcutaneously. Epo dosing is cumbersome, resulting in patient non-compliance and frequent, cyclical fluctuations in blood Epo and hematocrit values.
In light of the protective effects of Epo on the cardiovascular system, as well as the current challenges associated with recombinant Epo treatment, implantation of an Epo-eluting device may be an effective alternative to the current treatment modality. Such an implantable device might also better control hematocrit values and potentially even protect organ microvasculature from injury.
Recent studies focused on developing alternative Epo delivery systems are in progress. Investigators have demonstrated the feasibility of encapsulating recombinant Epo in different types of bioabsorbable polymers (See, e.g., Yeh et al., J. Microencapsulation, 2007; 24(1):82-93; Pistel et al., J. of Controlled Release, 1999; 59(3):309-325; Bittner et al., European Journal of Pharmaceutics an Biopharmaceutics, 1998; 45:295-305; Morlock et al., Journal of Controlled Release, 1998; 56:105-115). While encapsulation of peptides and small molecules into biodegradable envelopes can be achieved using several techniques, the encapsulation of proteins has associated challenges. For example, it has been difficult to obtain continuous Epo release profiles with minimal initial burst as well as sufficient protein loading within the microspheres. The development of a recombinant Epo-loaded, implantable device may require frequent drug-reloading or device replacement to ensure long-term, robust disease management.
Other investigators have placed less emphasis on recombinant Epo and are pursuing a genetic engineering and cell therapy approach. Naffakah et al. examined whether the secretion of Epo from genetically modified cells could represent an alternative to repeated injections for treating chronic anemia. Naffakh et al., Human Gene Therapy, 1996; 7(1):11-21. In this study, primary mouse skin fibroblasts were transduced with a retroviral vector in which the murine Epo cDNA was expressed under the control of the murine phosphoglycerate kinase promoter. These “Neo-organs” containing the genetically modified fibroblasts embedded into collagen gels were implanted into the peritoneal cavity of mice resulting in an increase in hematocrit and serum Epo concentrations after a 10-month observation period. The implantation of Epo-secreting fibroblasts represents a potential method for permanent in vivo Epo delivery.
Similarly, Orive et al. investigated the long-term functionality of an ex vivo gene therapy approach. Orive G et al., Molecular Therapy, 2005; 12(2):283-9. Polymer microcapsules loaded with Epo-secreting myoblasts were implanted into the peritoneum and subcutaneous tissue of syngeneic and allogeneic mice. High and constant hematocrit levels were maintained for more than 100 days in all implanted mice. Interestingly, the functionality of capsules implanted in the allogeneic mice persisted until day 210 after implantation. These results demonstrate the feasibility of cell encapsulation technology for the long-term delivery of Epo within an allogenic model.
In addition, many companies are also developing cell encapsulation technology. StemCells (CytoTherapeutics) is developing cell capsules that can be surgically implanted and release substances that cross the blood-brain barrier for neurological applications. Novocell Inc. (San Diego, Calif.) is developing encapsulated islet cells for insulin-dependent diabetes. Islet Technology, Inc. (St. Paul, Minn.) is also developing islet microencapsulation technology and has demonstrated the long-term persistence of their implants in a diabetic dog for more than 3 years. Amcyte Inc. (Santa Monica, Calif.) is developing islet cells to form an artificial pancreas using photocross-linkable alginate or polyethylene glycol capsule. Finally, MicroIslet Inc. (San Diego, Calif.) is developing a suspension of microencapsulated, porcine islets for injection into the abdominal cavity using a highly biocompatible alginate.
Indeed, several efforts exist, attempting to exploit cell and protein encapsulation as a means to deliver therapeutic agents. In total, the state-of-the-art has generated very compelling and useful data, and these efforts have demonstrated the utility of encapsulation as a method for the controlled, long-term delivery of Epo in vivo. However, there are considerable safety issues that must be resolved before the encapsulation of genetically modified cells can be utilized for therapeutic proposes.
There remains a need for implantable devices that overcome traditional problems associated with therapeutic deployment.